The present invention relates to a method for producing a conductive segment, and a conductive segment. More particularly, the invention relates to a method for producing a conductive segment used as an electrode of a resistance plate of a liquid level detector detecting a liquid level height of a transportation fuel tank of automobiles, aircraft and the like, and a conductive segment produced by the production method.
A liquid level detector in which a float arm slides on a resistance plate by a float moving up and down according to a liquid level is conventionally known as a liquid level detector detecting a liquid level height of a fuel tank of, for example, automobiles.
One example of a liquid level detector is described below. FIG. 1 is an electrical block diagram for explaining a structural example of a sensor used in a liquid level sensor. FIG. 2 is an explanatory view for explaining a structural example of a liquid level detector. FIG. 3 is an explanatory view for explaining a structural example of a variable resistor in a sensor.
A sensor 2 of a liquid level detector 1 is equipped with a variable resistor 3 changing a resistance value in the course of movement of contacts 19 and 20 described hereinafter in conjunction with height transition of a liquid level in the inside T of a fluid tight vessel, and the variable resistor 3 is connected to a fixed resistor 7 in series and connected to a power circuit 4 applying a given voltage to the variable resistor 3 and the fixed resistor 7.
As shown in FIG. 2 and FIG. 3, the sensor 2 includes a resistance plate 13 attached to a body frame 12, and a sliding body contactor 14 connected to one end of a float arm 11 having attached to other end thereof a float 10 floating on a liquid level by buoyancy of a liquid. The resistance plate 13 of the sensor 2 is provided with a first conductive pattern 15 and a second conductive pattern 16 as electrodes, and those two conductive patterns 15 and 16 are arranged in a state of being parallel to each other in an arc shape centering a rotation axis 21 of the float arm 11. An input-output conductive part 17 is connected to one end of the first conductive pattern 15, and an input-output conductive pattern 18 is connected to one end of the second conductive pattern 16.
The first conductive pattern 15 is constituted of a plurality of conductive segments 15a arranged at given intervals in a circumferential direction of the arc shape, and a resistor 15b electrically connecting the plurality of conductive segments 15a to each other. The second conductive pattern 16 is constituted of a plurality of conductive segments 16a arranged at given intervals in a circumferential direction of the arc shape, and a coupling body 16b electrically connecting a plurality of the conductive segments 16a to each other.
Two contacts 19 and 20 electrically connected to each other are provided on the sliding body contactor 14. Furthermore, a rotation axis 21 located at other end of the float arm 11 is connected to the sliding body contactor 14. The float arm 11 circles in an arrow Y direction of FIG. 3 centering the rotation axis 21 by that the float 10 floating on a liquid level moves in a lower direction from the position of a liquid level when filled up, according to the amount of consumption. The sliding body contactor 14 also turns in an arrow Y direction of FIG. 3 according to the turning of the float arm 11. By the turning motion of the sliding body contactor 14, the contacts 19 and 20 electrically contact while sliding on the respective conductive segments 15a and 16a arranged on the first conductive pattern 15 and the second conductive pattern 16. By this, length of the resistor 15b located in a circuit between the input-output conductive part 17 connected to the first conductive pattern 15 and the input-output conductive part 18 connected to the second conductive pattern 16 changes, and a resistance value of the circuit changes (that is, a resistance value of the variable resistor 3 in FIG. 1 changes). Thus, the variable resistor 3 is constituted of the first conductive pattern 15, the second conductive pattern 16 and the sliding body contactor 14.
When voltage is applied to the variable resistor 3, the sensor 2 detects potential difference between the input-output conductive parts 17 and 18 and outputs its output signal to a processing circuit 5, and the processing circuit 5 indicates a residual amount of a liquid to an indicator such as a meter 6 in analog display or bar graph display, based on an output signal of the sensor 2. In the meter 6, a fixed resistor may be arranged on a wiring to the processing circuit 5.
In the liquid level detector, a material of the contact generally uses a silver-palladium (AgPd) alloy, a silver-copper (AgCu) alloy, a silver-nickel (AgNi) alloy and the like. The conductive segment includes, for example, a mixture of a silver-palladium (AgPd) powder and a glass, and is obtained by mixing a silver powder, a palladium powder and a glass powder to obtain a paste, printing the paste on a resistance plate, drying the paste and then burning the paste.
The liquid level detector is sometimes used in a fuel tank of automobiles using an electrolyte (alcohol) such as ethanol or methanol, or gasoline containing the electrolyte, as a fuel. Silver (Ag) has small electric resistance and has excellent conductivity. However, contacts and conductive segments deteriorate or corrode by sulfur content, moisture, alcohol content and the like in a fuel, resulting in poor conduction. As a result, drawbacks such that measurement cannot be performed and incorrect values are obtained may occur. On the other hand, an insulating material is formed by sulfurization of a conductive segment itself and sulfurization of an abrasion powder generated by the sliding between conductive segments and contacts. This increases a resistance value and causes turbulence of output waveform, leading to deterioration of reliability of a liquid level detector.
Due to the current circumstances regarding fuels around the world, fuels having various formulations are frequently used, and it is necessary to provide a fuel gauge preventing the above obstacles and having reliability. In view of this, a technique of covering a part at which contacts of conductive segments slide, with an alloy containing gold (Au) in order to prevent deterioration and corrosion of the conductive segments and contacts is known (for example, see Patent Document 1, 2 and 3).
Patent Document 1: Japanese Patent Publication No. JP-A 2003-287456
Patent Document 2: Japanese Patent Publication No. JP-A 2009-162694
Patent Document 3: U.S. Pat. No. 6,681,628
However, in the inventions disclosed in PTL 1 and 2, the thickness of the covering layer of an alloy containing gold (Au) is decreased with the passage of time; therefore, effectiveness of the effect may not be sufficient. Furthermore, the inventions disclosed in PTL 1 and 2 had the problem that gold (Au) must be contained in a large amount in order to secure sufficient deterioration resistance and corrosion resistance, and this led to the problem of increase in costs.
Meanwhile, the conventional technologies disclosed in PTL 3 have effectiveness to corrosion and sulfurization, but have the problems that the effectiveness is not sufficient, and a large amount of gold (Au) is used, leading to the increase in costs.